Editing Williams tube

{{short description|Early form of computer memory}}
{{Use dmy dates|date=April 2022}}
[[File:James Pomerene IAS machine.jpg|thumb|upright| [[James Pomerene]] with a Williams–Kilburn tube, a 5CP1A cathode ray tube, used in the memory array of the [[IAS machine|IAS computer]] circa 1951 ]]
[[File:Williams tube.agr.jpg|thumb|upright|Williams–Kilburn tube from an [[IBM 701]] at the [[Computer History Museum]], in [[Mountain View, California]]]]
[[File:SWAC 003.jpg|thumb|Memory pattern on [[SWAC (computer)|SWAC]] Williams tube CRT]]
{{Memory types}}
The '''Williams tube''', or the '''Williams–Kilburn tube''' named after inventors [[Frederic Calland Williams|Freddie Williams]] and [[Tom Kilburn]], is an early form of [[computer memory]].<ref name="Kilburn-CCS">{{Citation | last = Kilburn | first = Tom | author-link = Tom Kilburn | title = From Cathode Ray Tube to Ferranti Mark I | journal = Resurrection | publisher = The Computer Conservation Society | volume = 1 | issue = 2 | year = 1990 | url = https://www.cs.man.ac.uk/CCS/res/res02.htm#e | issn = 0958-7403 | access-date = 15 March 2012 }}</ref><ref name="Napper-C50">{{Cite web | author=Brian Napper | date=25 November 1998  | title=Williams Tube |  url=https://curation.cs.manchester.ac.uk/computer50/www.computer50.org/kgill/williams/williams.html | publisher=University of Manchester | access-date=1 October 2016}}</ref> It was the first [[Random-access memory|random-access]] digital storage device, and was used successfully in several early computers.<ref>{{citation |title=Early computers at Manchester University |journal=Resurrection |volume=1 |issue=4 |publisher=The Computer Conservation Society |date=Summer 1992 |url=https://www.cs.man.ac.uk/CCS/res/res04.htm#g |issn=0958-7403 |access-date=7 July 2010}}</ref>

The Williams tube works by displaying a grid of dots on a [[cathode-ray tube]] (CRT). Due to the way CRTs work, this creates a small charge of [[static electricity]] over each dot. The charge at the location of each of the dots is read by a thin metal sheet just in front of the display. Since the display faded over time, it was periodically refreshed. It operates faster than earlier [[Delay-line memory#Acoustic delay lines|acoustic delay-line memory]], at the speed of the electrons inside the vacuum tube, rather than at the [[speed of sound]]. The system was adversely affected by nearby electrical fields, and required frequent adjustment to remain operational. Williams–Kilburn tubes were used primarily on high-speed computer designs.

Williams and Kilburn applied for British patents on 11 December 1946,<ref>[http://www.wikipatents.com/GB-Patent-645691/improvements-in-or-relating-to-electrical-information-storing-means GB Patent 645,691]</ref> and 2 October 1947,<ref>[http://www.wikipatents.com/GB-Patent-657591/improvements-in-or-relating-to-electrical-storage-apparatus GB Patent 657,591]</ref> followed by United  States patent applications on 10 December 1947,<ref>{{US patent|2951176}}</ref> and 16 May 1949.<ref>{{US patent|2777971}}</ref>

==Working principle==
{{More citations needed section|date=March 2016}}
The Williams tube depends on an effect called [[secondary emission]] that occurs on [[cathode-ray tube]]s (CRTs). When the electron beam strikes the [[phosphor]] that forms the display surface, it normally causes it to illuminate. If the beam energy is above a given threshold (depending on the phosphor mix) it also causes [[electron]]s to be struck out of the phosphor. These electrons travel a short distance before being attracted back to the CRT surface and falling on it a short distance away. The overall effect is to cause a slight positive charge in the immediate region of the beam where there is a deficit of electrons, and a slight negative charge around the dot where those electrons land. The resulting [[Potential well|charge well]] remains on the surface of the tube for a fraction of a second while the electrons flow back to their original locations.<ref name="Kilburn-CCS"/> The lifetime depends on the [[Electrical resistance and conductance|electrical resistance]] of the phosphor and the size of the well.

=== Writing ===
The process of creating the charge well is used as the write operation in a computer memory, storing a single binary digit, or [[bit]]. A positively charged dot is erased (filling the charge well) by drawing a second dot immediately adjacent to the one to be erased (most systems did this by drawing a short dash starting at the dot position, the extension of the dash erased the charge initially stored at the starting point). This works because the negative halo around the second dot will fill in the positive center of the first dot. A collection of dots or spaces, often one horizontal row on the display, represents a computer word. Increasing beam energy makes the dots bigger and last longer, but requires them to be further apart, since otherwise, nearby dots erase each other.

The beam energy has to be large enough to produce dots with a usable lifetime. This places an upper limit on the [[Areal density (computer storage)|memory density]], and each Williams tube could typically store about 256 to 2560 bits of data.

Because the electron beam is essentially inertia-free and can be moved anywhere on the display, the computer can access any location, making it a random access memory. Typically, the computer would load the [[memory address]] as an X and Y pair into the driver circuitry and then trigger a [[time base generator]] to sweep the selected locations, reading from or writing to the internal registers, normally implemented with [[flip-flop (electronics)|flip-flop]]s.

=== Reading ===
Reading the memory took place via a secondary effect caused by the writing operation. During the short period when the write takes place, the redistribution of charges in the phosphor creates an [[electrical current]] that induces [[voltage]] in any nearby [[Electrical conductor|conductors]]. This is read by placing a thin metal sheet just in front of the display side of the CRT. During a read operation, the beam first writes to the selected bit locations on the display. Those locations that were previously written to are already depleted of electrons, so no current flows, and no voltage appears on the plate. This allows the computer to determine there was a "1" in that location. If the location had not been written to previously, the write process will create a well and a pulse will be read on the sheet, indicating a "0".<ref name="Kilburn-CCS" />

Reading a memory location creates a charge well whether or not one was previously there, thus destroying the original contents of that location. So any read has to be followed by a rewrite to reinstate the original data. In some systems this was accomplished using a second electron gun inside the CRT that could write to one location while the other was reading the next.

=== Refreshing ===
Since the display fades over time, the entire display has to be periodically refreshed using the same basic method. As the data is read and then immediately rewritten, this operation can be carried out by external circuitry while the [[central processing unit]] (CPU) is busy carrying out other operations. This refresh operation is similar to the [[memory refresh]] cycles of [[Dynamic random-access memory|DRAM]] in modern systems.

=== Erasing ===
Since the refresh process caused the same pattern to continually reappear on the display, there was a need to be able to erase previously written values. This was normally accomplished by writing to the display just beside the original location. The electrons released by this new write would fall into the previously written well, filling it. The original systems produced this effect by writing a small dash, which was easy to accomplish without changing the master timers and simply producing the write current for a slightly longer period. The resulting pattern was a series of dots and dashes. There was a considerable amount of research on more effective erasing systems, with some systems using out-of-focus beams or complex patterns.

=== Visibility of data ===
Some Williams tubes were made from [[radar]]-type cathode-ray tubes with a [[phosphor]] coating that made the data visible, while other tubes were purpose-built without such a coating. The presence or absence of this coating had no effect on the operation of the tube, and was of no importance to the operators, since the face of the tube was covered by the pickup plate. If a visible output was needed, a second tube connected in parallel with the storage tube, with a phosphor coating, but without a pickup plate, was used as a display device.

==Development==
Developed at the [[Victoria University of Manchester|University of Manchester]] in [[England]], it provided the [[Computer program|program]] storage medium for the [[Manchester Baby]], the first electronic [[stored-program computer]], which first successfully ran a program on 21 June, 1948.<ref>{{Citation | last = Napper | first = Brian | title = Computer 50: The University of Manchester Celebrates the Birth of the Modern Computer | url = http://www.computer50.org/ | access-date = 26 May 2012 | url-status = dead | archive-url = https://web.archive.org/web/20120504133240/http://www.computer50.org/ | archive-date = 4 May 2012 }}</ref> Rather than the Williams tube memory being designed for the Baby, the Baby was a [[testbed]] to demonstrate the reliability of the memory.<ref>{{Citation|last1=Williams|first1=F.C.|last2=Kilburn|first2=T.|title=Electronic Digital Computers|journal=Nature|volume=162|pages=487|date=Sep 1948|doi=10.1038/162487a0|issue=4117|bibcode=1948Natur.162..487W |s2cid=4110351|postscript=.|doi-access=free}} Reprinted in ''The Origins of Digital Computers''</ref><ref>{{Citation|last1=Williams|first1=F.C.|last2=Kilburn|first2=T.|last3=Tootill|first3=G.C.|title=Universal High-Speed Digital Computers: A Small-Scale Experimental Machine|url=http://www.computer50.org/kgill/mark1/ssem.html|journal=Proc. IEE|date=Feb 1951|volume=98|issue=61|pages=13–28|postscript=.|doi=10.1049/pi-2.1951.0004}}</ref> [[Tom Kilburn]] wrote a 17-instruction program to calculate the highest [[proper factor]] of numbers as large as 2<sup>18</sup>. Tradition at the university has it that this was the only program Kilburn ever wrote.<ref>{{Harvnb|Lavington|1998|p=11}}</ref>

Williams tubes tended to become unreliable with age, and most working installations had to be hand tuned. By contrast, mercury [[delay-line memory]] was slower and not random access, as the bits were presented serially, which complicated programming. Delay lines also needed hand tuning, but did not age as badly and enjoyed some success in early digital electronic computing despite their data rate, weight, cost, thermal and toxicity problems. The [[Manchester Mark 1]], which used Williams tubes, was successfully commercialised as the [[Ferranti Mark 1]]. Some early computers in the United States also used Williams tubes, including the [[IAS machine]] (originally designed for [[Selectron tube]] memory), the [[UNIVAC 1103]], [[IBM 701]], [[IBM 702]] and the [[SWAC (computer)|Standards Western Automatic Computer]] (SWAC). Williams tubes were also used in the Soviet [[Strela computer|Strela-1]] and in the Japan TAC (Tokyo Automatic Computer).<ref>{{cite book|url=https://archive.org/details/bitsavers_onrASurveyomputers1953_8778395|title=A survey of automatic digital computers|last1=United States Office of Naval Research|date=1953|publisher=Office of Naval Research, Dept. of the Navy|page=[https://archive.org/details/bitsavers_onrASurveyomputers1953_8778395/page/n92 87]|language=en}}</ref>

<gallery class="center" heights=220 widths=220>
Williams-tube.jpg|A Williams–Kilburn tube
WilliamsTubeFigure1.tiff|Diagram of Williams tube memory from the 1947 patent
File:Museum of Science, Boston, MA - IMG 3160.JPG|SWAC Williams tube assembly
File:SEACComputer 004.jpg|Diagram of SWAC Williams tube module
</gallery>

==See also==
*[[Atanasoff–Berry computer]]&nbsp;– Used a type of memory called ''regenerative capacitor memory''
*[[Mellon optical memory]]
{{Clear}}

==References==
;Notes
{{reflist|30em}}

;Bibliography
{{refbegin}}
*{{citation | last=Kilburn | first=T. | year=1948 | title=A Storage System For Use With Binary Digital Computing Machines | type=Ph.D. thesis | location=University of Manchester | url=https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488439}}
*{{citation |last=Lavington |first=Simon |title=A History of Manchester Computers |year=1998 |edition=2nd |publisher=The British Computer Society |isbn=978-1-902505-01-5}}
{{refend}}

==Further reading==
*{{cite book |last=Bashe |first=Charles J. |title=IBM's Early Computers |publisher=[[MIT Press]] |year=1986 |isbn=0-262-02225-7 |url-access=registration |url=https://archive.org/details/ibmsearlycompute00bash }}
*{{cite book |last=Lavington |first=Simon H. |title=Early British Computers |publisher=[[Manchester University Press]] |year=1980 |isbn=0-932376-08-8}}
*{{cite book |last=Randell |first=Brian |author-link=Brian Randell|title=The Origins of Digital Computers |year=1982 |edition=3rd |publisher=Springer-Verlag |isbn=0-387-11319-3}}

==External links==
{{commons category|Williams tubes}}
*[https://web.archive.org/web/20030216135550/http://www.computer50.org/kgill/williams/williams.html The Williams Tube]
*[https://www.youtube.com/watch?v=SpqayTc_Gcw#t=0m59s Manchester Baby and the birth of Computer Memory]
*[https://frank.pocnet.net/sheets/201/6/6571.pdf RCA 6571 Computer storage tube data sheet]

{{Electronic components}}
{{Thermionic valves}}
{{Primary storage technologies}}

[[Category:Cathode ray tube]]
[[Category:History of computing hardware]]
[[Category:History of computing in the United Kingdom]]
[[Category:Department of Computer Science, University of Manchester]]
[[Category:Types of RAM]]
[[Category:Vacuum tubes]]
[[Category:Computer memory]]